Tamoxifen – a breast cancer medicine used by millions of women — won’t work when taken with some common antidepressants, a new study says.

Tumors were more than twice as likely to return after two years in women taking antidepressants like Prozac, Zoloft and Paxil, compared with those taking the anti-cancer drug alone, the study showed.

The research by Medco Health Solutions was presented Tuesday at a meeting of the American Society of Clinical Oncology in Orlando.

Doctors began treating hot flashes with antidepressants, an unapproved use, after a study linked the former standard remedy, hormone replacement therapy, to an increased risk of breast cancer and heart attacks. Hot flashes are also a side effect of tamoxifen.

Other types of antidepressants, such as Effexor, may be safer for women on tamoxifen, said Powel Brown, a cancer prevention expert at Baylor College of Medicine.